(Not Applicable)
(Not Applicable)
1. Field of the Invention
The invention relates to a process and a device for producing a brake drum or a brake disk comprising a carrier element made from a light metal alloy which is readily castable and machinable, and a friction lining made from a fiber- or particle-reinforced light metal alloy.
2. Related Art
An axle-mounted brake disk for rail vehicles is described in German Patent 44 00 898. The axle-mounted brake disk is produced as a monobloc disk or as a disk which is composed of a hub and friction ring, preferably with internal ventilation, and has hard ceramic particles dispersed in an aluminum alloy matrix. In this axle-mounted brake disk, it is intended that at least the friction ring, and preferably also the cooling ribs which are connected thereto, and/or also the hub should consist of a hypoeutectic AlSi-base alloy reinforced with hard ceramic particles. If the axle-mounted brake disk is comprised of a hub and friction ring, the components are screwed together.
Aluminum alloys which are reinforced with hard ceramic particles are very difficult to machine, however, and the solutions proposed in German Patent 44 00 898 require considerable machining. In the case of the monobloc disk, the hub, which also consists of the aluminum alloy reinforced with hard ceramic particles, has to be machined. In the disk which is composed of a hub and friction ring, the joint between the hub and the friction ring has to be machined. In both cases, this machining work is difficult to carry out owing to the aluminum alloy reinforced with hard ceramic particles which is used.
Accordingly, there is a need for a process and a device for producing a brake drum or a brake disk comprising a carrier element made from an inexpensive light metal alloy which is readily castable and machinable and has a wear-resistant friction lining made from a fiber- or particle-reinforced light metal alloy, and in which only the friction faces of friction linings must be machined.
EP 0 662 361 A1 describes a die-casting device for producing a metal object, having a ceramic filler pipe arranged in a bottom die, a ceramic filler plunger inside the filler pipe, a sliding annular seal between the filler pipe and the filler plunger, an induction coil which is positioned around the filler pipe in order to produce a pool of metal above the filler plunger, and a movable top die. In addition, vertical, insulated and water-cooled conductors are arranged in the filler pipe, so that the combined effect of the vertical conductors and the induction coil is to melt a slug which has been placed in the filler pipe with simultaneous turbulence and lifting of the molten material off the wall of the filler pipe.
The process according to this invention comprises:
introducing a predetermined quantity of casting material for the carrier element in the form of a slug of light metal alloy into a filler pipe of a die, and melting the slug therein;
producing the carrier element in a die by moving the molten light metal alloy from the filler pipe into the die;
cooling the molten material to a temperature at which it is dimensionally stable;
introducing a slug of light metal alloy for the friction lining into the filler pipe of the die, and melting the slug therein;
immediately afterwards, casting the light metal alloy for the friction lining onto the carrier element by moving the molten light metal alloy from the filler pipe into the die; and
cooling and removing the cast article.
According to either process described above, the materials may be introduced with the fill pipe in a molten state.
The process according to the invention makes it possible to produce a brake drum or a brake disk using a composite casting process. The fiber- or particle-reinforced light metal alloy friction lining is fixedly connected to the readily castable and machinable, as well as inexpensive, light metal alloy carrier element. Consequently, only the friction face of the friction lining requires special machining, while the machining steps for the carrier element do not entail any special costs and difficulties. Preferably, a short-fiber-reinforced light metal alloy can be used.
The carrier element or the friction lining can be produced in a first die, after which the casting-on or casting-around operation takes place in a second die.
Preferably, however, the carrier element or the friction lining is produced in a single die, with space created in this die for the friction lining or the carrier element, after the carrier element or the friction lining has cooled down, by moving parts of the die, in particularly core pullers, and the fiber- or particle-reinforced light metal alloy or the light metal alloy for the carrier element is introduced into this space.
A suitable hub structure ensures central access, which is optimum in terms of casting technology, for the metal which is to be cast on or around in the second cycle through the gates, which are easy to punch off.
In order to achieve a secure interlocking between the light metal alloy for the carrier element and the fiber and particle-reinforced light metal alloy during casting, projections, ribs and/or apertures may be provided, forming toothing between the light metal alloys which have been cast on or around one another.
It is particularly preferable if an intermetallic interlocking between the light metal alloy for the carrier element and the fiber- or particle-reinforced light metal alloy is produced during casting by means of a surface treatment carried out on the carrier element or the friction lining, this surface treatment preferably being exposure of the surface of the light metal alloy which is introduced into the die first to a flux, which may be present in a carrier fluid in the form of a suspension or a solution, immediately before the second light metal alloy is applied.
It is particularly preferable if the treatment with a flux is combined with the cooling of the light metal alloy which is introduced into the die first, by applying a flux-containing coolant, in particular flux-containing water, to this light metal alloy.
In order to reduce the oxidation of the surfaces of the carrier element and of the friction lining, a vacuum of 20 to 100 mbar absolute or protective gas comprising nitrogen or argon or the like can be applied to or introduced into the die shortly before casting.
Cooling of the light metal alloy which is introduced into the die first to a temperature at which it is dimensionally stable requires only small amounts of water, which evaporates immediately while the flux prepares the surface of this light metal alloy for casting on or casting around with the second light metal alloy. If necessary, compressed air may subsequently be applied to the cast surface. This allows the cooling and drying time to be reduced.
According to an advantageous casting process, individual slugs of a molten light metal alloy for the friction lining and fiber and particle reinforcement material may be introduced separately into a filler pipe on the die. The components may then be inductively heated, whereby the light metal alloy is mixed uniformly with the fiber and particle reinforcement material as a result of the turbulence in the melt brought about by the induction currents. Then, the mixed molten material may be introduced into the space which is provided for the friction lining. Therefore, it is not necessary to provide or obtain a composite light metal alloy which is already provided with fiber or particle reinforcement material, and one step in the overall process may be eliminated.
The fiber or particle reinforcement material is introduced into the light metal alloy by means of the molten flow when melting down the light metal alloy or by introducing the material into a turbulent flow of the melt, which is brought about by agitation from above or centrifugal force produced by the rotation or by it being injected through a channel in the casting plunger.
The casting process may be arranged in such a way that firstly, a slug of the light metal alloy for the carrier element is introduced into the filler pipe, and this slug is inductively melted and introduced into the space in the die which is provided for the carrier element. While the carrier element is cooling, space is created for the friction lining and new gates by moving core pullers. While this is being done, a slug of the light metal alloy for the friction lining and fiber or particle reinforcement material are introduced into the filler pipe, simultaneously or at different times, and the light metal alloy is inductively melted and is brought to the casting temperature and is mixed with the fiber or particle reinforcement material, simultaneously or successively. The melted and mixed material is then introduced into the space in the die which has been created for the friction lining.
When the thixocasting process is used, it is also possible to successively introduce a slug of the light metal alloy with thixotropic properties for the carrier element and a slug made from the light metal alloy for the friction lining with embedded fiber or particle reinforcement material into a filler pipe on the die. Then one or both slugs are is inductively heated to a temperature which lies above the solidus and below the liquidus, or to heat one of the slugs or both slugs to the melting temperature, and the heated material is pressed into the die in the pasty or molten state. In the case of light metal alloys without reinforcement material, the thixotropic state can be achieved by controlled cooling and holding in the solid-liquid two phase field and by simultaneous agitation. The agitation may be provided from above by means of a mechanical stirrer, or from below by means of the casting plunger in the filler pipe, or by the rotation of the filler pipe in the magnetic field inside the induction coil. By suitably designing a winding gradient of the induction coil, the vertical upward and downward movement of the shot sleeve containing the casting plunger in the coil is able to make the agitation movement more uniform given a constant temperature of the metal. By varying the induction frequency between 50 and 10,000 Hz, the stirring can be carried out in a controlled manner and in a very short time with moderate movement of the bath. The thixocasting process can be used particularly advantageously with a slug made from the light metal alloy for the friction lining with embedded fiber or particle reinforcement material, since this ensures that the homogeneity of the material for the friction lining is maintained. The slug of the light metal alloy for the carrier element may likewise be pressed into the die in the thixotropic state, but also in the molten state.
The device for producing a brake drum or brake disk according to this invention is comprised of a die having:
a ceramic filler pipe which is arranged in a bottom die made from steel;
a ceramic filler plunger in the filler pipe;
a sliding annular seal between the filler pipe and the filler plunger and/or a fixed seal between the bottom die and the filler pipe, made from heat-resistant fiber material and/or from graphite and/or from ceramic/metal composite material;
an induction coil which is positioned around the filler pipe and has a concentrated winding density in order to rapidly produce a pool of metal above the filler plunger;
a movable top die made from steel or molding sand; and
spaces in the bottom die and/or the top die for core pullers which make way for the carrier element or the friction lining.
By means of the induction coil, the light metal alloy for the carrier element and the fiber- or particle reinforced light metal alloy for the friction lining are heated to or held at the melting temperature or are heated to or held at a temperature which is above the solidus and below the liquidus, in order to press these alloys into the die cavity by means of the filler plunger.
The filler pipe and the filler plunger are comprised of ceramic material, in order not to produce any inductive heating in this area apart from in the light metal alloy which is situated inside the filler pipe.
The filler pipe containing the filler plunger is able to absorb verticsl, radical-concentric or radical-eccentric movements of linear or oscillating form on order to even out or optimize the efficency of the magnetic field inside the induction coil.
The spaces arranged in the bottom die and/or the top die for core pullers which make way for the carrier element or the friction lining are utilized when, after the first light metal alloy which was introduced into the die has cooled down, the second light metal alloy is introduced into these spaces which have been formed.
In order to control the cooling of the first light metal alloy introduced, the bottom die may preferably comprise a heated base and a cooled area which delimits the circumference of the casting, and the top die may also be heated. In view of the fact that the optimum casting temperature lies within a range from 550 to 750xc2x0 C., the base of the bottom die and the top die may be heated to approximately 300xc2x0 C., and the area which delimits the circumference of the casting may be cooled to 100xc2x0 C. This allows optimized central feed-through and peripheral, controlled solidification. Insulating layers with sealing elements may be arranged between the die elements in order to maintain these different temperatures.
The seal between the filler pipe and the bottom die can advantageously be effected by means of a sealing bead which engages in a corresponding recess in the bottom die.
The formation of an oxide skin when heating the light metal slug and on the light metal alloy is prevented by applying a vacuum or by means of a protective gas comprising nitrogen or argon in the filler pipe.
Applying pressure to the light metal alloy through the gate openings in the die by means of the filler plunger can be carried out in a particularly advantageous manner if the filler plunger has a convex base. This filler plunger may have a concentric channel for injecting metallic or ceramic powders, fibers or liquids. This filler plunger may also be used as a squeezing plunger at the end of the casting operation, but it is also advantageously possible to arrange a squeezing plunger in the top die, coaxially with respect to the filler pipe. The squeezing plunger may be of two-part design, with a coaxial, displaceable core area. A concentric annular piston may then advantageously be used to shear, break or punch off the sprue.
Since the base of the bottom die is heated and the area of the bottom die which delimits the circumference of the casting is cooled, it is advantageous if these two parts of the bottom dies are divided from one another and if the base and/or the area are additionally divided in the radial direction, thus facilitating separating the die and making heat transfer through an air gap more difficult.
If a vacuum is applied to the die, it is possible to provide seals which enable a vacuum at levels as low as less than 30 mbar to be maintained.
Spraying the light metal alloy which was introduced into the die first with coolant and flux can be carried out by raising the top die and then spraying from outside, but it is preferable for the die to have a built-in device for spraying coolant and/or flux. This makes it possible to spray only those surface areas of the light metal alloy introduced into the die first which come into contact with the light metal alloy which is introduced subsequently.